The present invention relates to temperature regulating devices and in particular to such devices utilized to control the temperature of the environment of an electronic circuit. More particularly, this invention relates to temperature regulating apparatus utilizing the heat developed by one or more semiconductor devices to control the temperature within an enclosure. With even greater specificity, this invention relates to temperature control for enclosures containing electronic devices or circuits having operating characteristics that are highly sensitive to temperature fluctuations.
Piezoelectric crystal electric oscillators, and other such devices and electronic circuits having operating characteristics that are highly sensitive to temperature are usually operated within an enclosure in which the temperature is regulated in order to isolate them from outside temperature fluctuations and their attendant effects.
In general, thermal regulating devices include three main parts:
a heating source producing heat by the action of an electric current, such heat being applied to the body or enclosure to be heated; PA1 a temperature sensitive element for measuring the temperature of the body or temperature within the enclosure to be heated and for developing an electric control voltage related to that measured temperature; and PA1 a circuit responsive to the electric control voltage thus generated, for regulating the electric current within the heating source.
Conventional temperature regulating apparatus perform adequately and achieve satisfactory results only when the temperature range outside of the enclosure is not extreme. Devices currently available are not able to regulate within highly defined tolerances when the environment temperature fluctuates over a wide range. Such devices are not able to compensate effectively and rapidly for accidental, inadvertent or unintended temperature changes within the enclosure, or fail to induce a uniform heat distribution (heat homogeneity).
One known device utilizes a power transistor for heating coupled to an electrical power source between collector and emitter and includes a resistor in series with the emitter to avoid the heat burst phenomena by limiting the current through the power transistor. This arrangement results in poor heat homogeneity and requires a very small mechanical enclosure in order to obtain regulation on the order of tenths of degrees centigrade for an environmental temperature range of -40.degree. C. to +70.degree. C.
Another known device utilizes a plurality of transistors for heat generation, coupled in parallel to an electrical power source. These transistors are distributed on or about the enclosure to be temperature controlled. Each such paralleled transistor is separately controlled by a signal applied to its base. Although this arrangement provides better heat homogeneity, it is difficult to provide coordinated control of the base of each of the transistors. It is also difficult to match a plurality of paralleled transistors so that they will exhibit similar heating characteristics.
In addition, conventional temperature regulators are highly sensitive to power supply voltage fluctuations.